1. Field of the Invention
The present invention relates to a communication device, and more particularly to a communication device provided with an attenuator that attenuates a gain of an input signal.
2. Description of the Related Art
In an ultra-wideband (UWB) communication system that performs communications by using a wideband and high-frequency signal, signals having a wide range of frequencies can be used to perform communications. Accordingly, in this UWB communication system, communications are performed while hopping frequencies used by respective communication devices. FIG. 1 schematically shows a state where a communication device 101 and a communication device 102 perform communications. For example, consider a case where the communication device 101 transmits a signal to the communication device 102 while hopping frequencies used. Here, assume that noise or the like is mixed in the signal from the communication device 101 due to an influence from another communication device to thereby deteriorate quality of the signal. Such a case may be found when a mobile communication terminal such as a cellular phone performs communications near any of the communication devices, for example. In that case, the communication device 102 detects the deterioration in quality of the signal transmitted from the communication device 101 by use of an amount of noise included in a received signal, and then transmits a signal indicating the information to the communication device 101. The communication device 101 acquires the information on the deterioration in quality of the transmitted signal from the signal that is transmitted from the communication device 102. Here, there is known a method of suppressing deterioration in quality of the signal by attenuating a gain of a signal to be transmitted in that case. In this method, the communication device 101 once attenuates the gain of the signal to be transmitted and then transmits the signal to be transmitted again to the communication device 102.
For this reason, each of the communication devices 101 and 102 is provided with an attenuator that is a circuit for attenuating the gain of the signal to be transmitted. Here, http://www.mwave-lab.jp/vr_att.htm (Non-patent Document 1) discloses a technique to include multiple attenuators and switch the attenuators by a switch to adjust the gain of a signal to be transmitted. Specifically, each of the communication devices 101 and 102 includes multiple attenuators having different amounts of attenuation of the gain of a signal. The communication devices 101 and 102 each select one of the attenuators as necessary to transmit a signal to be transmitted while attenuating the gain of the signal with an appropriate amount of attenuation.
Meanwhile, there is also known an attenuator disclosed in Japanese Unexamined Patent Application Publication No. Hei 6-334504 (Patent Document 1), which implements a concrete circuit configuration of the above-described attenuator. FIG. 2 shows the circuit configuration of the attenuator disclosed in Patent Document 1. This attenuator is a T-type attenuator formed of a circuit in which field effect transistors (FETs) 201 and 202 are connected in series and another FET 203 is connected thereto in shunt. When a path between a source and a drain of the FET 203 is conducted by adjusting a gate voltage, a signal inputted from an input terminal (IN) of the attenuator is shunted into a component that passes through the FET 202 and a component that passes through the FET 203. As a result, the gain of a signal outputted from an output terminal (OUT) is attenuated.
Here, it is known that the amount of attenuation of the gain of the attenuator changes depending on the frequency of the input signal. Specifically, the amount of attenuation of the gain of the attenuator has a frequency characteristic. The reason that the amount of attenuation of the gain of the attenuator has the frequency characteristic is that the respective circuit elements constituting the attenuator have parasitic element components. Referring to the FET 201 and the FET 203 in FIG. 2, it is apparent that the FETs 201 and 203 each have a parasitic capacitor and a parasitic resistor. The input signal to the attenuator is also shunted through this parasitic capacitor. Moreover, since impedance of the parasitic capacitor changes depending on the frequency of the input signal, an amount of current to be shunted through the parasitic capacitor changes depending on the frequency of the input signal to the attenuator. As the input signal to the attenuator leaks through the parasitic capacitor in an amount corresponding to the frequency of the input signal, an amount of current to be outputted from the attenuator also varies depending on the frequency of the input signal. If the amount of current to be outputted from the attenuator varies depending on the frequency of the input signal, the gain of the signal to be outputted from the attenuator also varies depending on the frequency. As a consequence, the amount of attenuation of the gain for the input signal to the attenuator has the frequency characteristic.
The present inventor has found out the following problems of the above-mentioned conventional techniques. In recent years, communications using a wireless USB (universal serial bus) applying a UWB communication system has been drawing attention. In the communications using the wireless USB, in accordance with the standard thereof, a difference between the amounts of attenuation of gains of each two attenuators included in a communication device needs to satisfy a range of 2 dB±1 dB within a frequency band used by the wireless USB. For example, assume a case where a communication device is provided with four attenuators ATT1 to ATT4 having different amounts of attenuation of gains and the communication device uses these attenuators ATT1 to ATT4 by switching these attenuators with one another. In this case, a difference between the amounts of attenuation of the gains that the attenuators ATT1 and ATT2 respectively have, a difference between the amounts of attenuation of the gains that the attenuators ATT2 and ATT3 respectively have, and a difference between the amounts of attenuation of the gains that the attenuators ATT3 and ATT4 respectively have need to satisfy the range of 2 dB±1 dB within the frequency band used by the wireless USB. However, as described previously, each attenuator has the considerably variable amount of attenuation of the gain depending on the frequency of the input signal. Accordingly, the difference in the amount of attenuation of the gains between each two attenuators is generally not constant within the frequency band used by the wireless USB. A concrete example is shown in FIG. 3. FIG. 3 shows frequency dependency of the amount of attenuation of the gain of the attenuator. The longitudinal axis indicates the amount of attenuation while the lateral axis indicates the frequency of the input signal. Meanwhile, a “to-be-used frequency band” in the graph indicates the frequency range used by the wireless USB. Here, a curved line C shows an attenuation characteristic of the attenuator ATT1 included in the communication device while a curved line D shows an attenuation characteristic of the attenuator ATT2 included in the communication device, for example. It is apparent that the curved line C and the curved line D have different change ratios for the change in the frequency of the input signal. As a consequence, the difference between the amount of attenuation of the gain of the attenuator ATT1 and the amount of attenuation of the gain of the attenuator ATT2 at a point A diverges considerably from the difference between the amount of attenuation of the gain of the attenuator ATT 1 and the amount of attenuation of the gain of the attenuator ATT2 at a point B. Accordingly, in the example shown in FIG. 3, the difference between the amounts of attenuation of the gains of the attenuators ATT1 and ATT2 is not constant in the to-be-used frequency band, the attenuator ATT1 having the attenuation characteristic indicated with the curved line C and the attenuator ATT2 having the attenuation characteristic indicated with the curved line D. If these attenuators ATT1 and ATT2 are used in the communication device based on the wireless USB, there arises a problem that the communication device does not meet the standards of the wireless USB.